1. Field of the Invention
The present invention is related to a method and apparatus for constructing an electrical contact or electrode, which may, for example, comprise a sparkplug.
2. Disclosure Information
Designers have long recognized the need for constructing electrical contacts or electrodes from composite materials. The need for such composites results from the motivation, on the one hand, to reduce the cost of the electrical contact or electrode by using a relatively cheaper conducting material for the bulk of the electrode, while maximizing, on the other hand, the durability of the electrode by introducing a more durable, but more costly, contact or tip material in the critical working area. Accordingly, U.S. Pat. No. 3,431,614 discloses a method for manufacturing an electrode having a metal core, a tubular metal sheath surrounding the core, and a welded metal cap enclosing the sheath. The metal cap is sheared from the material from which the cap is formed. It is possible, however, that electrodes may be produced according to this method which appear to be satisfactory but have caps which are not completely welded to the outer tubular sheath. This occurs because the shearing action will not seriously stress the bond between the metal cap and the tubular metal sheath. Separation of the tip from the balance of the electrode is a serious defect for any electrical component, but it is particularly serious with respect to automotive sparkplugs because a metallic object such as an electrode cap could cause destruction of an engine were the cap to become loose in the engine's combustion chamber.
A second type of electrode formation is disclosed in U.S. Pat. No. 3,976,240 in which an ultrasonic welding unit is employed for the purpose of welding a gold foil element onto a second metal member within the confines of a perforated metal plate. The balance of the gold foil element lying outside the welded portion is then stripped off through the shearing action of the perforated metal plate. This method suffers from a similar deficiency to that previously noted inasmuch as the shearing action employed for separating the balance of the non-welded contact material from the piece to which the contact is being applied may be conducted successfully, i.e., the shearing may occur successfully, even when the precious metal contact strip has not been properly applied to the electrode material.
Examples of methods for welding strips and pins together are shown in U.S. Pat. Nos. 2,784,297 and 3,891,789. Neither of these patents, however, are directed to a solution of the previously described problem regarding integrity of welding or bonding used for electrodes and electronic contacts.
U.S. Pat. No. 3,643,101 discloses a means for measuring the diameter of a small orifice based upon multiple detectors arranged to measure the intensity of radiation from a laser source emanating through the hole to be measured. This technique, as will be understood in view of this disclosure, may be employed with the method disclosed herein for assessing the quality of a welded electrode assembly.